Invoice tax now matches each line item's own rate

When you create an invoice, each line item is now taxed at the rate defined for that item, so the invoice total matches what the customer saw on the estimate.

Result / verify

1. Create an invoice with line items that have different tax rates.
2. Check the tax shown against each line.
3. Confirm the total equals the sum of each line's subtotal plus its own tax, and that it matches the originating estimate.